The present invention relates to direct-sequence spread spectrum radio communications systems, and particularly to systems using a short spreading sequence while still achieving low power density.
In standard radio communications, information transmission is accomplished by modulating a carrier signal in response to data. In spread spectrum communications, the carrier signal is additionally modulated by a spreading function. The advantages of spread spectrum communications include security, reduced interference, and compliance with Federal Communications Commission (FCC) Rules. Spread spectrum communication systems which comply with FCC Rules may operate without a license. Spread spectrum communications may be used for alarm systems, smoke detectors, utility metering systems, personal and automobile locators, and other applications involving many transmitters but few receivers.
FCC Rule 15.247 imposes four requirements on the spread spectrum communication system. First, the transmitter power cannot exceed one watt. Second, the receiver processing gain must be at least +10 decibels (dB). Third, the spectrum must be spread across at least a 500 kilohertz (kHz) bandwidth. Fourth, and most importantly for the purposes of the present invention, there is a maximum power density restriction. No more than +8 decibels above one milliwatt (dBm) of power may be concentrated within any three kilohertz bandwidth.
A typical spreading function is a pseudo-random pattern composed of a repeating sequence. The repeating sequence has a set number of periods or "chips" each of the same duration. During each period a phase, amplitude, or frequency modulation is applied to the signal. A pseudo-random sequence is selected so that the pattern appears to be noise. As used herein, the "length" of a pseudo-random sequence refers to the number of chips in the sequence, whereas the "duration" of a pseudo-random sequence refers to the amount of time for the communication system to pass through the sequence.
A typical transmitter of a pseudo-random signal generates a carrier, applies the pseudo-random modulation to the carrier, and then applies data modulation to the pseudo-randomly modulated carrier. As is well known in the art, one effect of the application of a pseudo-random sequence to the carrier signal is to spread the spectrum power over a wide band. A transmitter to be certified under FCC Rule 15.247 must spread signal power over at least 500 kHz. This may be accomplished by using chips shorter than about two microseconds, for a biphase form of spreading modulation.
A typical receiver for a pseudo-random signal uses a local oscillator to heterodyne the received signal to an intermediate frequency. By modulating the local oscillator signal with the same pseudo-random sequence, the spectrum spreading is cancelled, and the received signal fits into a relatively narrow bandwidth intermediate frequency filter. Such a system can meet the processing gain required by FCC Rule 15.247 with a pseudo-random sequence shorter than 63 chips, and possibly as short as 15 chips.
In order for the spectrum spreading to cancel and produce intelligible data, the pseudo-random sequence generated in the receiver must be time-synchronized with the sequence of the received signal. A typical procedure for acquiring synchronization is to step the starting point of the locally generated sequence through the range of the sequence while monitoring the amount of signal power coming through the narrow intermediate frequency filter. When the locally generated sequence is not synchronized, the spectrum remains spread, and only a small amount of the received signal power passes through the intermediate frequency filter. However, when the locally generated sequence is nearly synchronized to the correct timing, the spreading is cancelled, most of the received signal power fits into the narrow band of the intermediate frequency filter, and the monitored signal power increases by the processing gain of the system. After achieving a rough synchronization, the system stops stepping through the pseudo-random sequence, and makes smaller shifts to refine the accuracy of the synchronization.
If a spread spectrum communication system has many transmitters sending to one receiver, the signals from the various remote transmitters will generally not arrive with the same sequence timing. Therefore, the synchronization acquisition procedure has to be repeated at the start of each packet. This is accomplished by starting each packet with a preamble which does not contain any data modulation. The duration of the preamble is sufficient for the receiver to go through the worst-case synchronization search, adjust the local sequence to the correct timing, and receive the signal with full processing gain, before the start of any data modulation. If the preamble is too short and the receiver fails to synchronize before the data starts, then data errors will occur. This is a particular problem in communication systems which use time division multiple access to communicate short packets from many transmitters to one receiver.
From the point of view of communication efficiency, the preamble is wasted time. Therefore, it is desirable to use as short a duration for the preamble as possible while retaining reliable synchronization acquisition. The length of time required to acquire synchronization is approximately proportional to the length of the pseudo-random sequence. This is because, in the worst-case scenario, the acquisition process must step through the full length of the sequence before discovering the correct timing. Typically, the duration of each step is proportional to the length of the sequence, but this is not required. Thus, a short pseudo-random sequence can provide a short preamble time.
In addition to the rule regulating bandwidth spread and processing gain, the FCC imposes a spectral power density rule. FCC Rule 15.247 requires that the signal power must be evenly spread over the bandwidth so that no more than +8 dBm of power may be concentrated within any three kilohertz bandwidth. In the prior art, the maximum power density is approximately inversely proportional to the length of the pseudo-random sequence.
Two methods of compliance with the spectral power density rule have been used. First, the communication system may use a long pseudo-random sequence. For example, a sequence length on the order of 255 chips will spread the power sufficiently for a one watt transmitter to meet the +8 dBm rule. However, such a long pseudo-random sequence produces a long acquisition time and a very wasteful preamble.
Second, the communication system may use a low power transmitter. For example, if a 389 milliwatt transmitter is used, the power density rule will be met if the sequence length is about 63 chips. However, a low power transmission is more susceptible to interference. It would be preferable to use a high-power transmitter so that the receiver could be spaced further away.
Thus, according to the prior art, the power density rule creates a trade-off between lower power transmitters, with a higher susceptibility to interference, and longer sequence lengths, with reduced communication efficiency.
In view of the foregoing, it is an object of the present invention to provide a communication system having a short preamble and a high transmitter power, while still meeting the FCC power density rule.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims.